(1) Field of the Invention
The present invention relates to an optical transmitter utilizing an external modulator such as a Mach-Zehnder type optical modulator and an optical transmission system utilizing such an optical transmitter, and particularly to an optical transmitter and an optical transmission system for transmitting an optical signal added with a required optical wavelength chirp.
(2) Related Art
In an optical communications system having a large capacity over a long distance, it is required to reduce degradation of an optical signal when transmitting the same through a transmission path. It is known that a waveform distortion due to a self phase modulation, which is one of the degradation causes of optical signal, can be corrected at an optical transmitting terminal side by adding an optical wavelength chirp (hereinafter simply called xe2x80x9cchirpxe2x80x9d) to the optical signal. It is also known that the optimum value of such a chirp depends on power of optical signal to be transmitted and on wavelength dispersion of a transmission path. For example, a wavelength division multiplexing (WDM) transmission system using 30 waves requires mutually different optimum chirp amounts for optical signals in 30 channels, respectively.
Known as a conventional technique for adding a chirp to an optical signal is to utilize a Mach-Zehnder type optical modulator formed of lithium niobate (LiNbO3; hereinafter called xe2x80x9cLNxe2x80x9d), for example. There has been also proposed a technique to render a chirp amount to be variable, by driving the aforementioned type Mach-Zehnder optical modulator by two drive signals corresponding to bifurcated arms (optical waveguides), and by varying a ratio between amplitudes of the drive signals (see Japanese Unexamined Patent Publication Nos. 7-7206 and 9-80363, for example). Concretely, the chirp amount becomes 1 when the modulator is driven by setting a ratio between voltage amplitudes of the two drive signals to be 1:0 (i.e., only one of the drive signals is input), for example, and becomes 0 when the voltage amplitudes of the drive signals are equivalent to each other.
In the aforementioned conventional optical transmitters utilizing Mach-Zehnder type optical modulators, it is required to adjust the amplitudes of respective drive signals so as to optimize the chirp amount. In optimally adjusting the amplitudes of the drive signals, possible variations of time delays of respective signals require an adjustment of a phase difference between the two drive signals. However, this phase adjustment has been extremely laborious work, and difficult to practice.
The present invention has been carried out in view of the conventional problems as described above, and it is therefore an object of the present invention to provide an optical transmitter and an optical transmission system capable of readily conducting an adjustment of a chirp amount.
To achieve the object, with one aspect of an optical transmitter according to the present invention, there is provided an optical transmitter utilizing a Mach-Zehnder type optical modulator, the Mach-Zehnder type optical modulator including: a light input end for receiving light; a first arm and a second arm for branching the light from the light input end to propagate the branched light, respectively; a light output end for synthesizing the branched light propagated through the first and second arms to output the resultant light; a first electrode for applying a first drive signal to the first arm to thereby drive the first arm; and a second electrode for applying a second drive signal to the second arm to thereby drive the second arm, comprising amplitude adjusting parts for adjusting the respective amplitudes of the first and second drive signals; phase adjusting parts for adjusting the respective phases of the first and second drive signals; an amplitude controlling part for detecting the respective amplitudes of the first and second drive signals, to thereby feedback control the amplitude adjusting parts; and a phase controlling part for detecting the respective phases of the first and second drive signals, to thereby feedback control the phase adjusting parts.
In the optical transmitter having such a constitution, the light input into the light input end of the Mach-Zehnder type optical modulator is bifurcated to be propagated through the first and second arms. The respective lights propagated through the first and second arms are synthesized into a resultant light and thereafter, the resultant light is output from the light output end. This Mach-Zehnder type optical modulator is applied with the first and second drive signals to first and second electrodes, respectively, to thereby cause changes in phases of the respective lights propagated through the first and second arms, respectively, so that intensity modulations of the respective lights are conducted in accordance with the first and second drive signals, and simultaneously a chirp is added corresponding to a ratio between the amplitudes of the first and second drive signals. The amplitudes of the first and second drive signals are monitored by the amplitude controlling part, and feedback controlled by the amplitude adjusting part so that an amplitude ratio between the first and second drive signals becomes a value corresponding to the optimum value of a chirp amount. Further, the phases of the first and second drive signals are monitored by the phase controlling part and feedback controlled by the phase adjusting part so that these phases are brought into, for example, an antiphase relation and, then, the first and second drive signals are applied to the first and second electrodes, respectively. In this way, there can be realized an optical transmitter capable of readily optimizing the chirp to be added to an optical signal.
As a concrete constitution of the optical transmitter, the amplitude controlling part may detect the respective amplitudes of the first and second drive signals after propagated through the first and second electrodes, respectively, and the phase controlling part may detect the respective phases of the first and second drive signals after propagated through the first and second electrodes, respectively. Alternatively, the amplitude controlling part may detect the respective amplitudes of the first and second drive signals before being applied to the first and second electrodes, respectively, and the phase controlling part may detect the respective phases of the first and second drive signals before being applied to the first and second electrodes, respectively.
Further, when the optical transmitter comprises: low frequency signal superimposing parts, each of which superimposes a predetermined low frequency signal symmetrically on a xe2x80x9c1xe2x80x9d side and a xe2x80x9c0xe2x80x9d side of each of the first and second drive signals; and a drift controlling part for detecting a low frequency signal component included in the optical signal output from the Mach-Zehnder type optical modulator to thereby judge an occurring state of an operating point drift, and for controlling the operating point of the Mach-Zehnder type optical modulator so that the operating point drift is compensated for, it is preferable that the amplitudes of the low frequency signals superimposed on the first and second drive signals, respectively, are varied corresponding to an amplitude ratio corresponding to an optical wavelength chirp amount.
According to such a constitution, the amplitude of each low frequency signal to be superimposed on both sides of each of the first and second drive signals in order to detect the operating point drift is adjusted in accordance with the amplitude ratio corresponding to the optimum chirp amount, together with an amplitude of a main signal. In this way, in the sum signal of the first and second drive signals, a superimposition ratio of the low frequency signals becomes constant. Thus, even if the amplitude ratio between the first and second drive signals is changed when the setting of the chirp amount is changed, the low frequency signals to be detected at the drift controlling part becomes constant. As a result, even when the chirp amount is controlled by adjusting the amplitude ratio between the first and second drive signals, no affection is imposed on the detection and control of the operating point drift based on the superimposition of the low frequency signals.
Further, when the optical transmitter comprises: low frequency signal superimposing parts, each of which superimposes a predetermined low frequency signal on either one of a xe2x80x9c1xe2x80x9d side and a xe2x80x9c0xe2x80x9d side of each of the first and second drive signals; and a drift controlling part for detecting a low frequency signal component included in the optical signal output from the Mach-Zehnder type optical modulator to thereby judge an occurring state of an operating point drift, and for controlling the operating point of the Mach-Zehnder type optical modulator so that the operating point drift is compensated for; it is preferable that the amplitudes of the low frequency signals superimposed on the first and second drive signals, respectively, are kept constant independently of an amplitude ratio corresponding to an optical wavelength chirp amount.
According to such a constitution, the low frequency signals having constant amplitudes independent of the chirp amount setting are superimposed on one sides of the first and second drive signals, respectively. In this way, in the sum signal of the first and second drive signals, a superimposition ratio of the low frequency signals becomes constant. Thus, even if the amplitude ratio between the first and second drive signals is changed when the setting of the chirp amount is changed, the low frequency signals to be detected at the drift controlling part becomes constant. As a result, no affection is imposed on the detection and control of the operating point drift.
Further, in the optical transmitter, the Mach-Zehnder type optical modulator may include a light modulating part, which is connected serially to a preceding stage of the light input end or a latter stage of the light output end, so as to modulate the light input into the optical transmitter in a two staged manner.
In the optical transmitter having such a constitution, the optical signal input into the Mach-Zehnder type optical modulator is modulated by being propagated through the light input end, first and second arms, and light output end, and further modulated at the light modulating part. The optical signal as modulated in such a two staged manner is added with a chirp controlled to the optimum value at the time of the former light modulation, similarly to the aforementioned case. In this way, it becomes possible to transmit such as a high-speed optical signal in an RZ data format, and to readily adjust the optimum chirp amount.
With another aspect of the present invention, there is provided an optical transmitter utilizing an external modulator made up by serially connecting a Mach-Zehnder type optical modulator and an optical phase modulator, comprising: an amplitude adjusting part for adjusting an amplitude of a drive signal for driving the optical phase modulator; a phase adjusting part for adjusting a phase of the drive signal; an amplitude controlling part for detecting the amplitude of the drive signal and for feedback controlling the amplitude adjusting part so that the amplitude of the drive signal becomes a value corresponding to an optical wavelength chirp amount set to reduce transmittal degradation of an optical signal; and a phase controlling part for detecting the phase of the drive signal and for feedback controlling the phase adjusting part so that the phase is matched with a phase of a signal for driving the Mach-Zehnder type optical modulator. The external modulator may include a polarization scrambler instead of the optical phase modulator.
In the optical transmitter of such a constitution, the light input into the external modulator is intensity modulated at the Mach-Zehnder type optical modulator and phase modulated at the optical phase modulator, to thereby be added with the chirp. Since the chirp amount to be added to the optical signal at this time is varied corresponding to the amplitude of the drive signal for the phase modulation, the amplitude adjusting part is feedback controlled so that the amplitude of the drive signal monitored at the amplitude controlling part becomes a value corresponding to the optimum value of the chirp amount. Further, since the phase of the amplitude-adjusted drive signal is required to be matched with the phase of the drive signal for driving the Mach-Zehnder type optical modulator, the phase adjusting part is feedback controlled in accordance with the phase of the drive signal monitored by the phase controlling part. In this way, it becomes possible to conduct the adjustment of the optimum chirp amount, even in a constitution utilizing an external modulator made up by combining a Mach-Zehnder type optical modulator with an optical phase modulator.
The optical transmission system according to the present invention comprises: a plurality of optical transmitters for transmitting optical signals of different wavelengths, an optical multiplexer for multiplexing the optical signals from the optical transmitters to transmit the multiplexed optical signal to a transmission path; and an optical demultiplexer for demultiplexing the optical signal transmitted through the transmission path into optical signals of respective wavelengths; and a plurality of optical receivers for receiving and processing the optical signals of respective wavelengths demultiplexed by the optical demultiplexer; wherein the aforementioned optical transmitter according to the present invention is adopted as each of the plurality of optical transmitters; and in each of the plurality of optical transmitters, the setting of the optical wavelength chirp amount is adjusted based on receipt information transmitted from each of the optical receivers corresponding to the applicable wavelength of the applicable optical transmitter and corresponding to the wavelengths adjacent to the applicable wavelength.
According to the optical transmission system having such a constitution, the chirp amount to be added to the optical signal of each wavelength is adjusted to become the optimum value at each optical transmitter, while taking account of an influence on the adjacent wavelengths. In this way, the optimization of the chirp amount for the optical signal of each wavelength can be readily conducted, to thereby allow acquisition of an excellent transmission characteristic.
Other objects, features and advantages of the present invention will become more apparent from the following description of preferred embodiments when read in conjunction with the accompanying drawings.